The present invention relates to an X-ray image intensifier.
A conventional X-ray image intensifier (to be referred to as an I.I. hereinafter) comprises a cylindrical glass envelope, an Al input window provided at one end of the glass envelope, and a cylindrical glass output envelope having a bottom and arranged at the other end of the glass envelope. An input screen is arranged in the glass envelope so as to face the input window, and an output screen is located on the bottom surface of the output envelope. A focusing electrode is attached to the inner surface of the glass envelope, and a conical accelerating electrode is provided near the output envelope.
X-rays emitted from an X-ray source are transmitted through an object and are incident on the input screen of the I.I. The input screen has a visual field having a diameter of, for example, 9 inches. A transmitted X-ray image of the object is converted into a photoelectron image by the input screen. The photoelectron image is focused and accelerated by the focusing electrode and accelerating electrode. Then, the image is incident on the output surface, and converted by the output screen into a fluoroscopic image having a diameter of, for example, 20 mm.
A conventional input screen has a structure wherein a phosphor layer having upper and lower deposited layers containing cesium iodide as a matrix is formed on an aluminum base plate. A vapor source is an activated particulate phosphor formed of cesium iodide containing sodium iodide. The lower deposited layer has a thickness of 180 .mu.m upon deposition of the phosphor particles in an argon gas atmosphere at 1.3.times.10.sup.-2 Pa or more. The upper deposited layer has a thickness of 30 m or less upon deposition of the phosphor particles on the lower deposited layer at a high vacuum of 1.times.10.sup.-3 Pa or less. A transparent conductive film made of, for example, indium oxide is formed on the surface of the upper deposited film.
The input screen is built into an I.I. and baked at a vacuum. Thereafter, a photoemissive layer is formed on the input screen.
The photocurrent per unit dosage rate (to be referred to as input sensitivity hereinafter) of the image intensifier having the above construction was measured, while X-rays having 7 mm thickness of aluminium half value layer are radiated to the I.I. with being operated. As a result, the input sensitivity was found to be 4.0 nA/mR.multidot.min.sup.-1. The critical resolution of the I.I. was measured by using a resolution chart formed of a 100-.mu.m thick lead plate located at the center of the input window surface. The critical resolution was found to be 40 lp/cm.
In the above I.I., even when the X-ray dosage rate of an X-ray passed through the patient is about 20 .mu.R/sec, an output image from the I.I. can be fluoroscopically observed by a TV camera. However, since the X-ray dosage rate is low, the number of X-ray quanta is subjected to spatial and temporal fluctuations. These fluctuations cause image noise which interferes with diagnostic examinations. In order to reduce the image noise, the X-ray dosage rate must be increased. As a result, the patient is exposed to an increased X-ray dose, to cause the problem of safety.
The present inventors performed the following two tests.